Fission modes in the reaction Pb-208(O-18,f)

Results of fission fragment mass-energy distributions of the compound Th-226 nucleus formed in the sub-barrier fusion reaction O-18+Pb-208 at an energy of O-18 ions E-lab = 78 MeV are reported. The reaction has been studied twice using two different accelerators, and both sets of experimental data agree quite well. Performed analysis of the experimental data with the use of a new multicomponent method has shown that alongside the well-known modes, i.e., the symmetric (S) and two asymmetric modes standard-one and standard-two, a high-energy mode standard-three has manifested itself. The last named mode appears due to the influence of the close-to-sphere neutron shell with N approximate to 50 in the light fission fragment group. Theoretical calculations of the prescission shapes of the fissioning nuclei Th-224,Th-226 confirm this conclusion.